9.3.4 Completion Options

When completing file names, certain file names are usually ignored.
The variable completion-ignored-extensions contains a list of
strings; a file name ending in any of those strings is ignored as a
completion candidate. The standard value of this variable has several
elements including ".o", ".elc", ".dvi" and
"~". The effect is that, for example, ‘foo’ can complete
to ‘foo.c’ even though ‘foo.o’ exists as well. However, if
all the possible completions end in “ignored” strings, then
they are not ignored. Displaying a list of possible completions
disregards completion-ignored-extensions; it shows them all.

If an element of completion-ignored-extensions ends in a
slash (‘/’), it's a subdirectory name; then that directory and
its contents are ignored. Elements of
completion-ignored-extensions which do not end in a slash are
ordinary file names, and do not apply to names of directories.

If completion-auto-help is set to nil, the completion
commands never display a list of possibilities; you must type ?
to display the list.

Partial Completion mode implements a more powerful kind of
completion that can complete multiple words in parallel. For example,
it can complete the command name abbreviation p-b into
print-buffer if no other command starts with two words whose
initials are ‘p’ and ‘b’.

To enable this mode, use M-x partial-completion-mode, or
customize the variable partial-completion-mode. This mode
binds special partial completion commands to <TAB>, <SPC>,
<RET>, and ? in the minibuffer. The usual completion
commands are available on M-<TAB> (or C-M-i),
M-<SPC>, M-<RET> and M-?.

Partial Completion mode also extends find-file so that
‘<include>’ looks for the file named include in the
directories in the path PC-include-file-path. If you set
PC-disable-includes to non-nil, this feature is
disabled.

Icomplete mode presents a constantly-updated display that tells you
what completions are available for the text you've entered so far. The
command to enable or disable this minor mode is M-x
icomplete-mode.